The dot plot shows how many hours some students slept last night. What is the median number of hours?

There are $1+2+6+3+4+3=19$ dots, so the median is the 10th value. Counting from the left: 1, 3, 9 dots up to 7 hours, so the 10th dot is at 8 hours. 7 hours is the mode; 7.5 is the middle of the scale.